Signal selecting system for keyboard type electronic musical instrument with conductive and nonconductive switches to actuate key-gates

ABSTRACT

In a signal selecting system for a keyboard type electronic musical instrument comprising: (1) a plurality of tone generators, (2) a plurality of key-gates, (3) a plurality of normally conductive switching devices, (4) a plurality of normally nonconductive switching devices, and (5) a plurality of keyswitches. The plurality of tone generators generate tone signals having frequencies corresponding to the notes of a musical scale, and the plurality of key-gates are coupled to said tone generators, respectively. The normally conductive switching devices are connected in series to each other and, at one end, to a power supply and, at another end, to the ground. Each of said plurality of normally nonconductive switching devices has a control terminal, an input terminal and an output terminal connected respectively to an input terminal of a normally conductive switching device and a control terminal of a key-gate. The plurality of keyswitches are operated by the corresponding keys of the keyboard, and each of said keyswitches is connected, at one end, to a control terminal of a normally conductive switching device, and, at another end, to ground.

United States Patent [72] Inventors Tlkatmgu Nlhjlma;

3,446,904 5/1969 Brand et al. 84/116 X 3,488,515 1/1970 l-liyoshi 84/].01 X

Primary ExaminerD. F. Duggan Assistant Examiner-Ulysses Weldon Attorney-Wenderoth, Lind and Ponack ABSTRACT: In a signal selecting system for a keyboard type electronic musical instrument comprising: (1) a plurality of tone generators, (2) a plurality of key-gates, (3) a plurality of normally conductive switching devices, (4) a plur Tlho Soto, both of Osaka, Japan 121 AppLNo. 3,9;1

[22] Filed Jan. 19, 1970 [45] Patented June 22, 1971 [73] Assignee Mltsushlta Electric Industrial Co., Ltd.

Osaka, Japan [32] Priority Jan. 18, 1969, Jan. 18, 1969 l J P [31 44/4299 and 44/4301 'ality of normally nonconductive switching devices, and (5) a plurality of keyswitches. The plurality of tone generators generate tone signals having frequencies corresponding to the notes of a musical scale, and the plurality of key gates are coupled to said tone generators, respectively. The normally conductive switching devices are connected in series to each other and, at one end, to a power supply and, at another end, to the ground. Each of said plurality of normally nonconductive switching devices has a control terminal, an input terminal and an output terminal connected respectively to an input terminal of a normally conductive switching device and a control terminal of a key-gate. The plurality of keyswitches are operated by the corresponding keys of the keyboard, and each of said mamm nm m MH W 4 mw muuw WE G 1 YMC ..7' u wm Rm ONOA mMO Pum W 1 M o mmwm m mm m S mm a m mma m m m G a F m m m mvw R e n m m m m d C m "m R n om m 9 L D m 1m ERN J m m 1 SL0 s m L C" n AE CW1 L NEBT C C rv ws 3 mm STWSl U M H NW N U UH U keyswitches is connected, at one end, to a control terminal of a normally conductive switching device, and, at another end, to ground.

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SIGNAL SELECTING SYSTEM FOR KEYBOARD TYPE ELECTRONIC MUSICAL INSTRUMENT WITH CONDUCTIVE AND NONCONDUCTIVE SWITCHES 'IO ACTUATE KEY-GATES FIELD OF THE INVENTION This invention relates to a keyboard type electronic musical I instrument, and more particularly to a novel signal selecting system which is capable of selecting only one tone signal from the tone signals corresponding to the keys simultaneously depressed.

DESCRIPTION OF THE PRIOR ART A conventional keyboard type electronic musical instrument is provided with a conventional signal selecting system comprising many transfer-type keyswitches each of which has a break-contact, a make-contact and a movable-contact. These keyswitches are connected in series, in such a way that the movable-contact of a keyswitch is connected to the breakcontact of the subsequent keyswitch, the movable-contact of the subsequent keyswitch is connected to the break-contact of the further subsequent keyswitch, and so on. Tone generators which generate tone signals are connected to the make-contacts of the keyswitches respectively. The movable-contact of the last keyswitch is the output terminal. In an conventional keyboard type electronic musical instrument, said tone generators are arranged in a descending order of frequencies and said keyswitches are arranged in a descending order of the notes. At the output terminal, only the tone signal having the lowest frequency can be obtained from the tone signals corresponding to the keys simultaneously depressed. The selected tone signal is sounded through a tone-forming means, an amplifying means and an electroacoustic transducing means.

However, the conventional signal selecting system fails to work reliably-due to poor contacts. A tone signal selected by a keyswitch far from the last keyswitch must pass through many contacts of the keyswitches connected in series to arrive at the output terminal.

Therefore, a conventional signal selecting system should have only a small number of transfer-type keyswitches connected in series connection from the point of view of reliability. The more the number of the transfer-type keyswitches in series, the less the reliability of the signal selection.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an improved and more reliable signal selecting system which comprises a plurality of keyswitches of the make-type and selects only one tone signal such as the tone signal, having the lowest or the highest frequency from the tone signals corresponding to the keys simultaneously depressed.

A further object of the present invention is to provide a signal selecting system which is capable of sustaining the selected tone signal.

A further object of the present invention is to provide a signal selecting system capable of cancelling the tone signal which has been previously selected and sustained when the succeeding keys are depressed.

A further object of the present invention is to provide a signal selecting system which is able to avoid selecting an unwanted tone signal upon the release of the depressed keys.

A still further object of the present invention is to provide an improved keyboard type electronic musical instrument with a novel signal selecting system. These objects are accomplished by providing a signal selecting system according to the present invention, which comprises:

1. a plurality of tone generators which generate tone signals having frequencies corresponding to the notes of a musical scale;

2. a plurality of key-gates which are coupled to said tone generators and each of which has a control terminal;

3. a plurality of normally conductive switching devices which are connected in series with each other and, at one end, to a power supply and, at another end, to ground and each of which has a control terminal, an input terminal and an output terminal;

. a plurality of normally nonconductive switching devices each of which has a control terminal connected to the input terminal of a normally conductive switching device, an input terminal connected to the control terminal of a normally conductive switching device and an output terminal connected to the control terminal of a key-gate; and v I a plurality of keyswitches which are operable by the corresponding keys of the keyboard, and each of which is connected, at one end, to the control terminal of a normally conductive switching devices and, at another end, to ground. Thus, when one or more keys are simultaneously depressed, the normally conductive switching devices corresponding to the depressed keys are cut off and only the normally nonconductive switching device closest to said power supply along the normally nonconductive switching devices corresponding to depressed keys can be conductive so as to open the corresponding key-gate.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1, a plurality of tone generators 1, 2, 3, 4 and 5 generate tone signals having the frequencies f jl, f f and f according to a musical scale. Said tone generators are arranged in order of frequencies in such a way A plurality of key-gates 6, 7, 8, 9 and 10 are provided which have the input sides coupled to the respective tone generators l, 2, 3, 4 and 5 respectively. Said key-gates 6, 7, 8, 9 and 10 are provided with control terminals 152, 153, 154, and 156, respectively. When a control signal is supplied to one of the said control terminals of key-gates, for example, the control terminal 153, the key-gate 7 is opened, and the corresponding tone signal appears at the output side of the key-gate 7. Each of said key-gates 6, 7, 8, 9 and 10 comprises a capacitor 104, I05, 106, 107 and 108 respectively and atransistor 109, 110, 111, 112 and 113, respectively. These capacitors 104, 105, 106, 107 and 108 are used to sustain the tone signals. Said transistors 109, 110, I11, 1 12 and 113 are connected in parallel with said capacitors I04, 105, 106, 107 and 108, respectively. Said transistors 109, 110, 111, 112

and 113 normally act as variable resistors to determine the sustain length of a sustained tone signal and, at particular times, they act as switches to discharge the charge on said capacitors 104, 105, 106, 107 and 108 instantaneously by the control of the potential on the bases of said transistors 109, 110,111, ..,112 and 113.

The output sides of said key-gates 6, 7, 8, 9 and 10 are coupled to the input of a tone-forming means 11 which is capable of producing various tone colors by changing the waveform of tone signal. The output signal of said tone-forming means 11 .is amplified by an amplifying means 12 and is sounded through an electroacoustic transducer 13.

A plurality of normally conductive switching devices la, 16a, 17a, 18a and 19a are connected in series with each other. Each of said normally conductive switching devices a, 16a, 17a, 18a and 190 has a control terminal designated as C, an input terminal designated as "IN" and an output terminal designated OUT." The series connection of normally conductive switching devices 150, 16a, 17a, 18a and 19a is connected, at one end, i.e. at the input terminal to a power supply 83 through another switching device 14 which is normally conductive. Said series connection of normally conductive switching devices 15a, 16a, 17a, 18a and 19a is connected, at another end, i.e., at the output terminal, to ground.

A plurality of normally nonconductive switching devices 15b, 16b, 17b, 18b and 1912 have control terminals shown as C, input terminals designated as IN and output terminals designated as OUT." Said control terminals of normally nonconductive switching devices 15b, 16b, 17b, 18b and 19b are connected to the input terminals of normally conductive switching devices 15a, 16a, 17a, 18a and 190 respectively. The input terminals of normally nonconductive switching devices 15b, 16b, 17b, 18b and 19b are connected to the control terminals of normally conductive switching devices 15a, 16a, 17a, 18a and 19a, respec tively. The output terminals of normally nonconductive switching devices 15b, 16b, 17b, 18b and 19b are connected to the control terminals 152, I53, 154, 155 and 156 of said key-gates 6, 7, 8, 9 and 10, respectively.

A plurality ofkeyswitches 20, 21, 22, 23 and 24 are operated by the corresponding keys of the keyboard. Said keyswitches 20, 21, 22, 23 and 24 are normally opened and are closed when the corresponding keys are depressed, respectively. Each of said keyswitches 20, 21, 22, 23 and 24 is, at one end, connected to the respective control terminals of normally conductive switching devices 15a, 16a, 17a, 18a and 19a and, at another end, to a bus bar 25. Said bus bar 25 is connected, through a switching means 80 and a resistor 82, to ground. Said switching means 80 is operated by a pedal 81. When said pedal 81 is depressed, said switching means 80 becomes conductive.

When one or more keys, for example, the keys corresponding to the keyswitches 21, 22 and 23 are simultaneously depressed, and said pedal 81 is depressed, the normally conductive switching devices 160, 17a and 180 corresponding to the depressed keys are cut off and only the normally nonconductive switching device 16b among the normally nonconductive switching devices 16b, 17b and 18b corresponding to the depressed keys becomes conductive because a potential difference develops only across said control terminal and said input terminal of normally nonconductive switching device 16b because it is the only nonconductive switching device among those corresponding to the depressed keys which is connected to the power supply 83. Then said normally nonconductive switching device 161; supplies a control signal to the control terminal 153 of the corresponding key-gate 7 and opens said key-gate 7.

Where said tone generators l, 2, 3,....., 4 and 5 are arranged in an ascending order of frequencies,

and the keys corresponding to said keyswitches 20, 21, 22, ....,23 and 24 are arranged in an ascending order of the notes, because the positions of said normally conductive switching devices 15a, 16a, 17a, 18a and 19a are respectively farther away from said power supply 83, this signal selecting system has a signal selecting action which selects the tone signal having the lowest frequency from the tone signals corresponding to the depressed keys.

Where said tone generators l, 2, 3, 4 and 5 are arranged in a descending order of frequencies,

and the keys corresponding to said keyswitches 20, 21, 22, 23 and 24 are arranged in a descending order of the notes, because the positions of said normally conductive switching devices 15a, 16a, 17a, 18a and 19a are respectively farther away from said power supply 83 this signal selecting system has a signal selecting action which selects the tone signal having the highest frequency from the tone signals corresponding to the depressed keys.

In order to cancel the tone signal which has been sustained at one of the key-gates 6, 7, 8, 9 and 10, all the bases of the transistors I09, 110, Ill, 112 and 1130f said keygates are connected to an output terminal designated OUT" of a cancelling system 26. An input terminal of said cancelling system 26 designated [N" is connected to the junction point of the switching means and the resistor 82. When one or more keys are depressed simultaneously, or when the position of the depressed key corresponding to the selected tone signal is changed, the current flowing through the resistor 82 changes. Said cancelling system 26 detects only the moment when the keys are depressed, or the moment when the position of the key corresponding to the selected tone signal is changed, and cancels the tone signal which has been previously selected and sustained by changing the bias potential on the base of the respective transistor. When the tone signal which has been selected and sustained is not canceled, said sustained tone signal and the newly selected tone signal will form an unwanted tone with an undesirable beat frequency.

In order to stop the aforesaid signal selecting action after a given short time period upon the release of the depressed keys, it is necessary to include a clamping system in the signal selecting system. Said clamping system, which is enclosed by a dotted line in FIG. 1 comprises a plurality of differentiating means 34, 35, 36, 37 and 38, a plurality of rectifying means 44, 45, 46, 47 and 48, a clamping pulse generator 27 and the aforesaid switching device 14. The output terminals of said normally nonconductive switching devices 15b, 16b, 17b 18b and 19b are connected to the respective input terminals of said plurality of differentiating means 34, 35, 36, 37 and 38, the output terminals of which are connected to the respective rectifying means 44, 45, 46, 47 and 48. Such a connection makes it possible to detect the moment when the depressed keys are released. An input terminal of said clamping pulse generator 27 is connected to a junction of said plurality of rectifying means 44, 45, 46, 47 and 48. Said switching device 14 is inserted between the aforesaid power supply 83 and aforesaid input terminal of the normally conductive switching device 15a and is normally conductive. The control terminal of said switching device 14 is connected to an output terminal of said clamping pulse generator 27. Said clamping pulse generator 27 generates a switching pulse with a given short time period upon the release of the depressed keys. Said switching device 14 is made nonconductive by said switching pulse, and the signal selecting action is stopped during said short time period. If the signal selecting action is not stopped during a given short time period upon the release of the depressed keys, an unwanted tone signal will be selected upon the release of said keys because the corresponding keyswitches are apt not to be simultaneously opened. It is quite impossible to open two or more keyswitches exactly at the same time.

The depression of the pedal 81 makes the aforesaid switching means 80 conductive. The signal selecting system is workable during a time period when said pedal 81 is depressed. Therefore, it is possible to control the signal selecting action by the depression of said pedal 81. Of course, the same controlling effect can be achieved by the insertion of said switching means 80 between said power supply 83 and said input terminal of normally conductive switching device 15a.

The aforesaid plurality of normally conductive switching devices 15a, 16a, 17a, 18a and 19a of FIG. 1 can be replaced by a plurality of transistors 29a, 30a, 31a, 32a and 33a as shown in FIG. 2 in which parts which are the same as in FIG. 1 are designated by the same, reference numbers. Each of said transistors 29a, 30a, 31a, 32a and 330 has a base, a collector and an emitter which respectively correspond to the control terminal, the input terminal and the output terminal of said normally conductive switching devices a, 16a, 17a, 18a and 19a. The resistors 74, 75, 76, 77 and 78 are connected across the bases and the collectors of said transistors to secure the normal conduction. The collector of the transistor 29a is connected to the power supply 83 through a transistor 28 corresponding to said switching device 14 of FIG. 1 and the emitter of the transistor 33a is connected to the ground through the resistor 79. In this embodiment of FIG. 2, the power supply 83 is designed to supply a voltage of l 5v.

Said plurality of normally nonconductive switching devices 15b, 16b, 17b, 18b and 19b of FIG. 1 can be replaced by a plurality of transistors 29b, 30b, 31b, 32b and 33b. Each of said transistors 29b, 30b, 31b, 32b and 33b has a base, a emitter and a collector which respectively correspond to the control terminal, the input terminal and the output terminal of said normally nonconductive switching devices 15b, 16b, 17b, 18b and 19b. Said bases of transistors 29b, 30b, 31b, 32b and 33b are connected to the collectors of said transistors 29a, 30a, 31a, 32a and 330 through the resistors 54, 55, 56, 57 and 58, respectively. Said collectors of transistors 29b, 30b, 31, 32b and 33b are connected through the resistors 59, 60, 61, 62 and 63, respectively to another power supply 95. In this embodiment of FIG. 2, said power supply 95 is designed to supply a voltage of l0v. Said collectors of transistors 29b, 30b, 31b, 32b and 33b are also connected to the control terminals 152, 153, 154, I55 and 156 of the key-gates 6, 7, 8, 9 and 10 through a plurality of diodes 39, 40, 41, 42 and 43, respectively. Each of said transistors 29b, 30b, 31b, 32b and 33b is normally nonconductive because there is no emitter current path. Said diodes 39, 40, 41, 42 and 43 are also normally nonconductive.

Said transistors 29a, 30a, 31a, 32a and 33a are supplied, at the collectors, with a reverse-biasing potential from voltage supply 96 through the resistors 49, 50, 51, 52 and 53 so as not to interfere the normal conduction, whereby the signal selecting action is reliably carried out. In the embodiment of FIG. 2, said reverse-biasing potential is designed to be +l5v.

FIG. 3 is a detailed circuit diagram of the key-gate as one example. Referring to FIG. 3, in which similar reference characters designate the components similar to those of FIG. 1 and FIG. 2 transistors 128, 129 and 130 act as the gates of the 4', 8' and 16 tone signals, respectively, and their bases are connected to the control terminal 153 of this key-gate 7 through a resistor 134b. The control terminal 153 is connected to the l.5v. bus through the resistor 139. The emitters of the transistors 128, 129 and 130 act as inputs terminals for said gates for the 4', 8' and 16 tone signals. The collectors are connected to the output buses, 149, 150 and 151 through the resistors 136, 137 and 138, respectively. These output buses are connected to the +4v. terminal through the resistors 140, 141 and 142, respectively, and through the resistor 143. The capacitor 144 is a decoupling capacitor. The emitters of the transistors are connected to the 2.0v. bus through biasing resistors 13], 132 and 133.

When the transistor 30b of FIG. 2 becomes conductive, the diode 40 which has been reversely biased, becomes conductive because of the potential rise at the anode, and the control signal is fed through the resistor 134b to the bases of the transistors 128, 129 and 130 and to the capacitor 105 for charging this capacitor for sustaining the corresponding tone signals. The charging time constant determines the rising time of the key-gate. When, thus, these gates are opened, the 4', 8' and 16 tone signals pass through the transistors 128, 129 and 130 and the resistors 136, 137 and 138, and enter the output buses 149, 150 and 151, respectively.

Upon removal of the control signal for opening the corresponding key-gate 7, the diode 40 becomes nonconductive and the key-gate 7 is isolated from the collector of transistor 30b. The key-gate 7 does not close instantaneously because of the charge of the capacitor 105. The capacitor is shunted by the resistors 135, 134a and 134b and the transistor which is in the active region. The base of this transistor 110 is connected to the sustain control bus 147 which is connected to a terminal 103 through a diode 102. This diode 102 is normally conductive. The controlling of the potential of the terminal 103 enables the control of the resistance value of the transistor 110.

The charge of the capacitor 105 is discharged through the resistors 135, 134a and 134b and the transistor 110. As this charge is discharged, the gates, i.e. the transistors 128, 129 and 130, close gradually and, therefore, the corresponding 4', 8 and 16 tone signals are sustained. This discharging time constant determines the sustain length of the tone signals. Consequently, the control of the potential of the terminal 103 makes possible the control'of the sustain length of the tone signal.

Again referring to FIG. 2, the portion corresponding to the aforesaid cancelling system will be explained. A change in the current flowing through the resistor 82 is amplified and clipped by the transistors 84 and 85 in order to get a much steeper step, and is differentiated by the differentiating means comprising the capacitor 87 and the resistor 88 in order to produce a pulse which is rectified by the rectifying means 89 in order to leave only the pulse corresponding to the moment when the keys are depressed. This remaining pulse, i.e. the rectified pulse triggers the monostable multivibrator comprising the transistors 92 and 98 so as to generate a switching pulse with a given short time period.

A transistor 100 is used as a switching device. The base of said transistor 100, is connected to the collector of the transistor 92 through a resistor 99; the emitter is connected to the cathode of said diode 102; and the collector is connected to ground. This transistor 100 works in association with the diode 102. This transistor 100 is normally nonconductive and the diode 102 is normally conductive. However, when said monostable multivibrator generates a switching pulse, this transistor 100 becomes conductive during said short time period of said switching pulse and the diode 102 becomes nonconductive. This causes all the transistors 109, 110, 111,

112 and 113 of the key-gates to become saturated and results in the discharge of the capacitors 104, 105, 106, 107 and 108 of the key-gates 6, 7, 8, 9 and 10 within this short time period. In this case, said transistors 109, 110, 111, 112 and 113 act as switches to discharge said capacitors 104, .105, 106, 107 and 108, respectively. Therefore, even if some tone signal has been previously selected and has been sustained, this tone signal is cancelled within this short time period and any newly selected tone signal is sounded. Said short time period is determined by the discharging time constant of the capacitor 93 of said monostable multivibrator. It is preferable to select said short time period to be between 3 and 5 milliseconds in accordance with the present invention.

Further, the aforesaid clamping system of FIG. 1 is also included in the embodiment of FIG. 2. The differentiating means corresponding to the aforesaid plurality of differentiating means 34, 35, 36, 37 and 38 consist essentially of capacitors 69, 70, 71, 72 and 73 and resistors 64, 65, 66, 67 and 68 respectively.

When, for example, the closed keyswitches 21, 22 and 23 are opened almost simultaneously, said transistor 30b, becomes nonconductive and the potential across the resistor 60 is removed. This potential change is differentiated by the capacitor 70 and the resistor 65 and rectified by the rectifying means 45. This rectified pulse corresponding to the moment when the depressed keys are released is amplified by the transistor 115 and triggers, through the diode 116, the monostable multivibrator comprising the transistors 119 and 126. Said monostable multivibrator, when triggered, generates a switching pulse with a given short time duration.

The switching device 14 of FIG. 1 can be replaced by a transistor 28. The base of said transistor 28 is connected to the collector of the transistor 126 through the resistor 125, the emitter is connected to said power supply 83, and the collector is connected to the collector of the transistor 29a. The transistor 126 is normally conductive and said transistor 28 is also normally conductive. However, when said monostable multivibrator generates said switching pulse, the transistor 28 also becomes nonconductive during said short time period, being reversely biased by the resistors 124 and 125. There is no power supply at the collector of the transistor 29a, and, therefore, the signal selecting action is stopped during said short time period. This short time period is determined by the discharging time constant of the capacitor 120 of said monostable multivibrator. It is preferable to select said short time period to be between 8 and 12 milliseconds in accordance with the present invention.

The signal selecting system of the present invention is, therefore, capable of selecting reliably the tone signal having the lowest or the highest frequency from the tone signals corresponding to the keys simultaneously depressed, even if the system is composed of a plurality of keyswitches of a makecontact type which is easily available.

It is possible to'increase the number of the keyswitches up to thirty or more in accordance with the present invention, and still the signal selecting system works reliably.

The signal selecting system of the present invention can be employed in combination with a conventional electronic organ. This signal selecting system, particularly when used as the pedal bass section of an electronic organ, is very effective. In this case, if the keyswitches of this system are operated by the keys of the manual keyboard, only one pedal is necessary instead of a plurality of pedal keys. It can be operated more easily than the keys of a conventional pedal keyboard. Also, in this case, it is preferable to adopt the signal selecting system capable of selecting the tone signal having the lowest frequency from the tone signals corresponding to the simultaneously depressed keys of the manual keyboard.

It is thought that the invention and its advantages will be un derstood from the foregoing description and it is apparent that various changes may be made in the form, construction and arrangement of theparts without departing from the spirit and scope of the invention or sacrificing its material advantages, the forms hereinbefore described and illustrated in the drawings being merely preferred embodiments thereof.

What 1 claim is:

l. A signal selecting system for a keyboard type electronic musical instrument comprising:

1. a plurality of tone generators which generate tone signals having frequencies corresponding to the notes of a musical scale;

2. a plurality of key-gates corresponding to the tone generators which are coupled to said tone generators and each of which has a control terminal;

3. a power supply;

4. a plurality of normally conductive switching devices corresponding to the key-gates and which are connected in series with each other and, at one end of the series connection, to said power supply and the other end of the series connection being grounded; and each of said normally conductive switching devices having a control terminal, an input terminal and an output terminal;

5. a plurality of normally nonconductive switching devices corresponding to the normally conductive switching devices and each of which has a control terminal connected to an input terminal of a corresponding normally conductive switching device and an output terminal connected to the control terminal of a corresponding keygate; and

6. a plurality of keyswitches adapted to be operable by the corresponding keys of a keyboard, and each of which is I connected, at one end, to the control terminal of a corresponding normally conductive switching device and being grounded at the other end, whereby when one or more keys are simultaneously depressed, the normally conductive switching devices corresponding to the depressed keys are cut off and only the normally nonconducting switching device closest to said power supply among the normally nonconductive switching devices corresponding to the depressed keys is conductive so as to open the corresponding key-gate.

2. A signal selecting system as claimed in claim 1, wherein said tone generators are arranged in an ascending order of frequencies corresponding to keys arranged in an ascending order of the notes, with the position, of the corresponding normally conductive switching devices being farther away from said power supply, so that only the tone signal having the lowest frequency is selected from the tone signals corresponding to the keys simultaneously depressed.

3. A signal selecting system as claimed inclaim 1, wherein said tone generators are arranged in a descending order of frequencies corresponding to keysarrangedin a descending order of the notes, with the position of the corresponding normally conductive switching devices being farther away from said power supply so that only the: tone signal having the highest frequency is selected from the tone signals corresponding to the keys simultaneously depressed.

4. A signal selecting system as claimed in claim I, wherein said each of a plurality of normally conductive switching devices consists of a transistor having a base acting as said control terminaL'a collector acting as said input terminal and a emitter acting as said output terminal.

5. A signal selecting system as claimed in claim 4 further comprising a source of reverse biasing potential coupled to the collector of each of normally conductive switching devices, whereby the normal conduction is not interferred with. t

6. A signal selecting system as claimed in claim 1, wherein said each of a plurality of normally nonconductive switching devices consists of a transistor having a base acting as said control terminal, an emitter acting as said input terminal, and a collector acting as said output terminal. 40 7. A signal selecting system as claimed in claim 1 wherein a diode is connected between each of the output terminals of said normally nonconductive switching devices and the control terminal of the corresponding key-gate, which diode becomes conductive when said normally nonconductive switching device becomes conductive.

8. A signal selecting system as claimed in claim 1 further comprising a cancelling system having an output terminal connected to all of said key-gates, and an input connected to said 5 keyswitches detecting only the moment when the keys are depressed and for thereupon cancelling the tone signal which has been selected.

9. A signal selecting system as claimed in claim 8 further comprising a resistor connected between the input terminal of said cancelling system and the junction between said keyswitches and ground. 5 10. A signal selecting system as claimed in claim 8 wherein said cancelling system comprises a differentiating means for producing pulse, a rectifying means coupled to the rectifying means for providing only a pulse corresponding to the moment when the keys are depressed, and a monostable multivibrator coupled to the rectifying means for generating a switching pulse with a given short time period, in order to cancel said selected tone signal within said short time period.

11. A signal selecting system as claimed in claim 1 further comprising a clamping system coupled to said switching devices and said power supply which detects the moment when the depressed keys of the keyboard are released and stops the signal selecting action during a given short time period upon the release of the depressed keys.

12. The signal selecting system as claimed in claim ll wherein said clamping system comprises a clamping pulse generator for generating a switching pulse with a short time period upon the release of the depressed keys and a switching device coupled between said power supply and said series con- LII nected normally conductive switching devices and connected to said clamping pulse generator to interrupt the power supply for stopping the signal selecting action during said short time period.

13. A signal selecting system as claimed in claim 12 in which said clamping system further comprises a plurality of differentiating means coupled to the output terminals of said normally nonconductive switching devices and a plurality of rectifying means coupled to corresponding differentiating means and to said clamping pulse generator.

14. A signal selecting system as claimed in claim 1, wherein each of said key-gates comprises a parallel connection of at least one capacitor for sustaining said tone signal and at least 

1. A signal selecting system for a keyboard type electronic musical instrument comprising:
 1. a plurality of tone generators which generate tone signals having frequencies corresponding to the notes of a musical scale;
 2. a plurality of key-gates corresponding to the tone generators which are coupled to said tone generators and each of which has a control terminal;
 3. a power supply;
 4. a plurality of normally conductive switching devices corresponding to the key-gates and which are connected in series with each other and, at one end of the series connection, to said power supply and the other end of the series connection being grounded; and each of said normally conductive switching devices having a control terminal, an input terminal and an output terminal;
 5. a plurality of normally nonconductive switching devices corresponding to the normally conductive switching devices and each of which has a control terminal connected to an input terminal of a corresponding normally conductive switching device and an output terminal connected to the control terminal of a corresponding key-gate; and
 6. a plurality of keyswitches adapted to be operable by the corresponding keys of a keyboard, and each of which is connected, at one end, to the control terminal of a corresponding normally conductive switching device and being grounded at the other end, whereby when one or more keys are simultaneously depressed, the normally conductive switching devices corresponding to the depressed keys are cUt off and only the normally nonconducting switching device closest to said power supply among the normally nonconductive switching devices corresponding to the depressed keys is conductive so as to open the corresponding key-gate.
 2. a plurality of key-gates corresponding to the tone generators which are coupled to said tone generators and each of which has a control terminal;
 2. A signal selecting system as claimed in claim 1, wherein said tone generators are arranged in an ascending order of frequencies corresponding to keys arranged in an ascending order of the notes, with the position of the corresponding normally conductive switching devices being farther away from said power supply, so that only the tone signal having the lowest frequency is selected from the tone signals corresponding to the keys simultaneously depressed.
 3. A signal selecting system as claimed in claim 1, wherein said tone generators are arranged in a descending order of frequencies corresponding to keys arranged in a descending order of the notes, with the position of the corresponding normally conductive switching devices being farther away from said power supply so that only the tone signal having the highest frequency is selected from the tone signals corresponding to the keys simultaneously depressed.
 3. a power supply;
 4. a plurality of normally conductive switching devices corresponding to the key-gates and which are connected in series with each other and, at one end of the series connection, to said power supply and the other end of the series connection being grounded; and each of said normally conductive switching devices having a control terminal, an input terminal and an output terminal;
 4. A signal selecting system as claimed in claim 1, wherein said each of a plurality of normally conductive switching devices consists of a transistor having a base acting as said control terminal, a collector acting as said input terminal and a emitter acting as said output terminal.
 5. A signal selecting system as claimed in claim 4 further comprising a source of reverse biasing potential coupled to the collector of each of normally conductive switching devices, whereby the normal conduction is not interferred with.
 5. a plurality of normally nonconductive switching devices corresponding to the normally conductive switching devices and each of which has a control terminal connected to an input terminal of a corresponding normally conductive switching device and an output terminal connected to the control terminal of a corresponding key-gate; and
 6. a plurality of keyswitches adapted to be operable by the corresponding keys of a keyboard, and each of which is connected, at one end, to the control terminal of a corresponding normally conductive switching device and being grounded at the other end, whereby when one or more keys are simultaneously depressed, the normally conductive switching devices corresponding to the depressed keys are cUt off and only the normally nonconducting switching device closest to said power supply among the normally nonconductive switching devices corresponding to the depressed keys is conductive so as to open the corresponding key-gate.
 6. A signal selecting system as claimed in claim 1, wherein said each of a plurality of normally nonconductive switching devices consists of a transistor having a base acting as said control terminal, an emitter acting as said input terminal, and a collector acting as said output terminal.
 7. A signal selecting system as claimed in claim 1 wherein a diode is connected between each of the output terminals of said normally nonconductive switching devices and the control terminal of the corresponding key-gate, which diode becomes conductive when said normally nonconductive switching device becomes conductive.
 8. A signal selecting system as claimed in claim 1 further comprising a cancelling system having an output terminal connected to all of said key-gates, and an input connected to said keyswitches detecting only the moment when the keys are depressed and for thereupon cancelling the tone signal which has been selected.
 9. A signal selecting system as claimed in claim 8 further comprising a resistor connected between the input terminal of said cancelling system and the junction between said keyswitches and ground.
 10. A signal selecting system as claimed in claim 8 wherein said cancelling system comprises a differentiating means for producing pulse, a rectifying means coupled to the rectifying means for providing only a pulse corresponding to the moment when the keys are depressed, and a monostable multivibrator coupled to the rectifying means for generating a switching pulse with a given short time period, in order to cancel said selected tone signal within said short time period.
 11. A signal selecting system as claimed in claim 1 further comprising a clamping system coupled to said switching devices and said power supply which detects the moment when the depressed keys of the keyboard are released and stops the signal selecting action during a given short time period upon the release of the depressed keys.
 12. The signal selecting system as claimed in claim 11 wherein said clamping system comprises a clamping pulse generator for generating a switching pulse with a short time period upon the release of the depressed keys and a switching device coupled between said power supply and said series connected normally conductive switching devices and connected to said clamping pulse generator to interrupt the Power supply for stopping the signal selecting action during said short time period.
 13. A signal selecting system as claimed in claim 12 in which said clamping system further comprises a plurality of differentiating means coupled to the output terminals of said normally nonconductive switching devices and a plurality of rectifying means coupled to corresponding differentiating means and to said clamping pulse generator.
 14. A signal selecting system as claimed in claim 1, wherein each of said key-gates comprises a parallel connection of at least one capacitor for sustaining said tone signal and at least one transistor acting as a variable resistor as well as a switch for discharging or charging said capacitor instantaneously.
 15. A signal selecting system as claimed in claim 1, further comprising a switching means coupled between said keyswitches and the ground in order to control the signal selecting action.
 16. A signal selecting system as claimed in claim 1, further comprising a switching means coupled between said power supply and said series connected one end of said normally conductive switching devices in order to control the signal selecting action. 